st: Doubt about xtivreg,first differences

["Rafael Terra de Menezes" <rflterra@hotmail.com>]

Dear Stata users,


I'm usin the command xtivreg, fd and some doubts arose while working with 
it. First, I ran the following:

xtivreg y w y z (l.y=l(2).y), fd

My panel has T=10 and the output shows T=7. I know first differencing the 
equation sweep out one period, and because there is a lag dependent 
variable, two years are swept out. I would like to know if the fact that I'm 
using yt-2  as instrument makes me lose another year.

Another question:  Is there any test to run after xtivreg,fd (test for 
validity of instruments for example)? And how can I deal heteroskedasticity 
in xtivreg?

Thanks,
Rafael Terra de Menezes
University of S�o Paulo - Brazil
Ms Student


*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/